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This happened to me and it turned out that despite me running the relevant command to delete all Time Machine snapshots, one remained. Listing the snapshots in Terminal, and then deleting the specific remaining snapshot by name, allowed me to partition the disk.
 
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My advice and my opinion only:
DON'T use BootCamp UNLESS you need a Windows solution that needs to be "booted natively".

If you want to run Windows on the Mac, use an "emulation solution" instead.
It could be "pay-for" software such as Parallels or VMWare fusion.
Or, try the free "Virtual Box" software.
 
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